Edexcell ppt Biology 2.39-2.43 Used in lessons to scaffold class teaching and as a revision resource for students

1. GAS EXCHANGE IN PLANTS
2.39 understand gas exchange (of carbon dioxide and oxygen) in relation to respiration and photosynthesis
CO2 and O2 diffuse in and out of leaves through stomata.
CO2 is used in photosynthesis and produced
by respiration, whereas O2 is used in
respiration and produced in photosynthesis!

2. Photosynthesis & Respiration
2.39 understand gas exchange (of carbon dioxide and oxygen) in relation to respiration and photosynthesis
Both processes run all the time. So the net amount of
glucose the plant produces (i.e. the amount it gets to
use for growth etc) is governed by the formula;
Net Glucose = Total production – Amount used in respiration
The Compensation Point is defined as;
0 = Total production – Amount used in respiration
Or
Photosynthesis = Respiration

3. Light Intensity & Gas Exchange
2.40 understand that respiration continues during the day and night, but that the net exchange of carbon dioxide and oxygen
depends on the intensity of light (TA)
The amount of glucose the plant uses in respiration in
nearly constant.
However, glucose production by photosynthesis is not.
It is dependent on the rate-limiting factors (i.e. light
intensity, CO2 level, water availability, temperature etc).
At night photosynthesis is virtually zero.
(Net Carbon Dioxide production)
In the day the photosynthesis is large.
(Net Oxygen production)

4. Light
Intensity &
Gas
Exchange
2.40 understand that respiration
continues during the day and night,
but that the net exchange of carbon
dioxide and oxygen depends on the
intensity of light (TA)

5. Leaf Structure and Photosynthesis
2.41 explain how the structure of the leaf is adapted for gas exchange

6. Stomata and Gas Exchange
2.42 describe the role of stomata in gas exchange
In sunlight the guard cell becomes turgid
Turgid guard cells open the stoma
Increases gas exchange
Low light causes guard cells to become flaccid
Flaccid guard cells close the stoma
Decreases water loss

7. Stomata and Gas Exchange
2.42 describe the role of stomata in gas exchange
Exp
1
Exp
2

8. Stomata and Gas Exchange
2.42 describe the role of stomata in gas exchange
Potometer
1) You must cut the shoots under water and you must
assemble the potometer under water. If air gets into the
xylem vessels of the plant, it can form air locks which will
prevent the plant taking up water and so prevent steady
transpiration.
1) Check all seals are airtight – coat seals with Vaseline jelly
3) The potometers should be left for the leaves to dry.
Alternatively dry the leaves gently with a paper towel. The
potometer will not work properly until any excess water
on the leaves has evaporated or been removed.
2) Adding food colouring to the water makes it easier to see
the air bubble in the capillary tube.

9. Stomata and Gas Exchange
2.42 describe the role of stomata in gas exchange

10. Experiment to Know
2.43 describe experiments to investigate the effect of light on net gas exchange from a leaf, using hydrogen-carbonate indicator (TA)
An experiment which will show the effect of light intensity on the
rate of gas exchange.
- Seal two leaves (still attached to the plant) in separate plastic
bags with some bicarbonate indicator solution.
- One of the bags is black and the other is translucent.
- The leaf in the black bag produces CO2 via respiration and the
colour of the bicarbonate indicator changes quickly to yellow.
- The leaf in the translucent bag produces O2 via photosynthesis
and the bicarbonate indicator solution changes to red slowly.
Bicarbonate Indicator colours:
Red in the absence of CO2
Yellow in the presence of CO2